Metal tubing is commonly made in a conventional tubing mill by feeding a strip of metal into a hollow cylindrical shoe with the longitudinal axis of the metal strip aligned with the axis of the shoe and the finished tubing. The welded seam is straight down the side of the tubing, aligned with the axis of the tubing as well. This requires that the width of the metal strip being formed is substantially equal to .PI. times the diameter of the tubing being made (.PI.D). For example, for 2 inch tubing these conventional tubing mills require a strip of metal 3.14.times.2 inches=6.28 inches in width. Tubing mills for manufacturing such tubing are complex and costly.
It is also known to manufacture helical seamed tubing wherein a narrow metal strip is fed at an angle into a cylindrical shoe to form a hollow tube with a helical seam which is then welded. An apparatus and method for forming helical seamed tubing with a diameter of 0.5 to 2 inches is disclosed in U.S. Pat. No. 4,501,948 to Yampolsky et al. The Yampolsky patent is directed to controlling the tendency of the seam to separate once it leaves the cylindrical shoe, the control being necessary in order to maintain the seam closed in such a manner that it can be effectively welded.
Cylindrical helical seamed tube wherein the seam thereof is at an angle of less than 25 degrees with respect to the axis of said tube is not known in the prior art.
In prior art helical seamed tubing, the strip of metal used to form the tubing is substantially narrower than that needed to form straight seam tubing and the angle between the longitudinal axis of the tubing being formed and the longitudinal axis of the metal strip being fed into the cylindrical dye is relatively large. In the Yampolsky device this angle is 33 degrees. This angle corresponds to the angle between the welded seam of the tubing and the axis of the tubing. At angles less than this, the edges of the metal strip are not effectively formed and result in flat spots and bulges along the edge which cannot be effectively welded.
Helical seamed tubing with this large angle of 33 degrees between the respective axes requires a much longer seam and therefore substantial welding. For a 10 foot length of tubing, the Yampolsky device requires a weld that is approximately 11.923 feet long, an increase of 19.23% in weld length compared to a straight seam tubing. This increased length of welding is costly and causes high temperatures in the tubing and distortion.
As the seam angle increases the tube has reduced strength as well which is undesirable in most applications. A tube with a seam angle that was less than 25 degrees would be stronger, and a tube with a seam angle approaching one degree would have very similar strength to a conventional straight seam tube.
Metal coil is conventionally manufactured in standard widths and when customers order a particular width there is a left over strip which is available at a much reduced price, since its uses are very limited. In a conventional straight seam 2 inch tubing mill, any strips of material that are less than approximately 6.28 inches in width are not suitable to form the material. These narrower strips can be purchased very cheaply, since there are few uses and therefore little demand for them.
A cylindrical helical seamed tube with a low helix angle and strength approaching that of a conventional straight seam tube would be desirable, as the cost of metal to make same would be reduced.
Such a tube, and a method and apparatus of making same from a strip of metal that is only slightly narrower than .PI.D would be advantageous, as the helical seam would be relatively long as opposed to conventional helical seamed tubing, thereby requiring only slightly more welding than the conventional straight seam tube. For example with an angle between the longitudinal axis of the tubing being formed and the longitudinal axis of the metal strip being fed into the cylindrical dye of only 10 degrees, a 10 foot length of tubing requires a weld that is only approximately 10.15 feet long, an increase of only 1.5% in weld length compared to a straight seam tubing.
Such a method an apparatus that allowed for forming a tube of a particular diameter from various widths of material would be particularly advantageous, as it would increase the sources of suitable material, and decrease the cost of the metal strip.